Apparatus for electronically measuring the angle of rotation of the polarization plane of a linearly polarized light beam produced by passage of the beam through a magneto-optical element subjected to a magnetic field to be measured

ABSTRACT

An arrangement for measuring the current flow through a conductor includes a magneto-optical element provided with a coil through which the current to be measured is passed. A beam of linearly polarized light passed through the magneto-optical element has its plane of polarization rotated by an amount proportional to the magnetic field produced by the coil, and the light beam after issuing from the magneto-optical element is split by an optical divider into two partial beams having different directions. One of the partial beams is passed through a polarizing filter to a photo-detector and the other beam is also passed through a polarizing filter but which has a passthrough direction rotated by an angle of about 45* relative to that of the other polarizing filter. The respective electrical outputs from the photo-detectors are then fed to multipliers to which sinusoidal signals from a local oscillator are also fed, the outputs from the multipliers are then added and the output from the adding member is then applied to a frequency demodulator.

United States Patent J aecklin [54] APPARATUS FOR ELECTRONICALLYMEASURING THE ANGLE OF ROTATION OF THE POLARIZATION PLANE OF A LINEARLYPOLARIZED LIGHT BEAM PRODUCED BY PASSAGE OF THE BEAM THROUGH AMAGNETO-OPTICAL ELEMENT SUBJECTED TO A MAGNETIC FIELD TO BE MEASURED 72]Inventor: Andre Jaecklin, Ennetbaden, Switzerland [73] Assignee:Aktiengesellschaft Brown, Boveri &,

Cie, Baden, Switzerland [22] Filed: Dec. 11, 1970 [21] Appl. No.: 97,181

[30] Foreign Application Priority Data Dec. 23, 1969 Switzerland..l9071/69 [52] US. Cl. ..324/96, 250/225, 356/117 [51] Int. Cl. ..G0lr19/00 [58] Field of Search ..250/209, 225; 356/116, 117; 324/96;356/114, 115

[56] References Cited OTHER PUBLICATIONS Ingersoll and James, ASensitive Photoelectric Method for Measuring the Faraday Effect, Review51 Sept. 19,1972

of Scientific MS., Vol. 24, No. 1, Jan. 1953, pp. 23- 25 Wild APhasemeter for Photoelectric Measurement of Magnetic Fields, Review ofScientific MS., Vol. 24, No. 8, Aug. 1970, pp. 1163- 1167.

An arrangement for measuring the current flow through a conductorincludes a magneto-optical element provided with a coil through whichthe current to be measured is passed. A beam of linearly polarized lightpassed through the magneto-optical element has its plane of polarizationrotated by an amount proportional to the magnetic field produced by thecoil, and the light beam after issuing from the magneto-optical elementis split by an optical divider into two partial beams having differentdirections. One of the partial beams is passed through a polarizingfilter to a photodetector and the other. beam is also passed through apolarizing filter but which has a pass-through direction rotated by anangle of about 45 relative to that of the other polarizing filter. Therespective electrical outputs from the photo-detectors are then fed tomultipliers to which sinusoidal signals from a local oscillator are alsofed, the outputs from the multipliers are then added and the output fromthe adding member is then applied to a frequency demodulator.

5 Claims, 2 Drawing Figures a 1 APPARATUS FOR ELECTRONICALLY MEASURINGTHE ANGLE OF ROTATION OF THE POLARIZATION PLANE OF A LIN EARLY POLARIZEDLIGHT BEAM PRODUCED BY PASSAGE OF THE BEAM THROUGH A MAGNETO- OPTICALELEMENT SUBJECTED TO A MAGNETIC FIELD TO BE MEASURED This inventionrelates to an improved and arrangement for the electronic measurement ofthe angle of rotation of the plane of polarization of a linearlypolarized light beam produced during passage of the light beam through amagneto-optical element subjected to a magnetic field to be measuredwherein the light beam, following its passage through the magnetoopticalelement, is split into two partial beams which are then linearlypolarized in different directions and I conducted to two photo detectorsconnected to separate transmission channels leading to an electronicmeasuring system.

It is known to modulate linearly, or circularly polarized light by meansof a magneto-optical element subjected to a magnetic field to bemeasured, i.e., crystals which are optically activated by a magneticfield extending parallel to the direction of the light ray beam, forexample, flint glass, or yttrium-iron garnets, by rotation of the planeof polarization and then by demodulating the wave to draw conclusions asto the modulating magnetic field. In this manner, the light beam can beutilized as an information carrier so that a contact-free measurement ispossible, for example, even of high voltages which no longer permit useof conventional measuring transformers because of difficultiesencountered inproviding a satisfactory insulation.

Specifically, it is known (Rev. Gen. de LElectr. July- August, 1967 p.1046) to conduct a linearly polarized light ray beam through amagneto-optical element surrounded by a current-carrying coil, and toarrange following this element an analyzer rotated by 45 relative to thepolarizer. The intensity of the ray beam following the analyzer is thenmeasured with a photo-multiplier and is given by the equation where I,is the magnitude of the light intensity ahead of the analyzer and 4) theangle by which the plane of polarization of the light is rotated afterpassing through the optically active element. As can be seen from thisequation, the relation between the angle of rotation 4) and the measuredlight intensity is not linear, which results either in adiflicult-to-plot or a complicated electronic measuring system. Anotherdisadvantage of this method is that fluctuations in intensity of thelight source from which the light beam is produced, and alsoinstabilities in the electronic measuring system falsify the results ofthe measurement.

Another known mode of measurement, as described at pages 1,057 ff. ofthe above-noted publication endeavors to eliminate these disadvantagesby connecting an additional magneto-optical element following thedescribed magneto-optical element whose optical activity is so orientedand regulated that the preceding rotation of the polarization plane iscompensated. The current necessary to effect this compensation serves asthe measuring quantity. Such a compensation method is better than thefirst one mentioned, but has the disadvantage that a too long responsetime is required, or a too high control power becomes necessary.

It is also known (IEEE J. of On. El. QE-2, 255 if. and 589 ff.) to splitthe light beam, following its passage through the magneto-opticalelement, by means of a Glan-Thomson-prism, the two crossed directions ofoscillations of which are aligned symmetrically to the plane ofoscillation of the unrotated light, into two partial light beams withmutually perpendicular planes of oscillation and then to conduct eachpartial beam to a corresponding photo-detector whose output is connectedto the input of a corresponding differential amplifier. This measuringmethod is not sufficiently sensitive, however, and in addition it issusceptible to trouble.

It has therefore been suggested to split the light beam by aKoesters-double prism before it passes through the magneto-opticalelement, and to measure the intensity of the resulting interferencebands. Two bands which are phase-shifted by can then be selected by twoslit diaphragms, and their respective intensities transmitted tocorrespondingly associated photo-detectors. Two signals which arephase-shifted by 90 are thus available for the electronic valuedetermining system. This has a general advantage, as is known fromhighfrequency engineering technique, that amplitudedisturbances causedby interferometer disturbances or the transmission channels in the caseof subsequent mixing and frequency demodulation of the signals enterconsiderably reduced into the measurement. Moreover, the phase-shiftedsignals have the advantage for an electronic evaluating system that aparticularly accurate FM-signal can be derived when they are mixed withthe frequency of a local oscillator. (Swiss patent 433,065). This methodhas the disadvantage, however, that the selection of the 90phase-shifted bands by means of slit diaphragms results in light losses.

The object of the present invention is to so improve upon the abovementioned modes of measurement that one has at his disposal forintroduction into the electronic evaluation system suitablephase-shifted signals which provide a high resolution and lowsusceptibility to troubles.

This objective is achieved, in accordance with the invention bypolarizing the two partial beams in planes of oscillation inclined by anangle of about 45 to each other. The orientation of the directions ofoscillation to the plane of oscillation of the unrotated light is irrelevant. Preferably, however, the direction of oscillation of one partialbeam is parallel to that of the unrotated light'The partial beams are asfar :as possible equal in their intensity.

In the case where the direction of oscillation of a partial beam isparallel to that of the unrotated light, the measurement according tothe present invention results in the following situation.

If A, is the amplitude of the first partial wave and A, that of thesecond partial wave, the amplitude of the wave after the first polarizeroriented parallel to the original polarization direction is equal to Acos and that of the wave after the second polarizer is equal to beingthe angle of rotation of the plane of polarization of the light effectedby the magneto-optical element. The intensity measured by thephoto-detectors is I proportional to the square of the amplitude, andthe electric currents supplied are therefore:

'i, z (11 /2) 1 +Sin 24 The electronic evaluating system thus has twoparallel signals available which are phase-shifted by 90, i.e., thesignals are in quadrature.

A frequent error of a magneto-optical element of the above-describedtype lies in a depolarization of the light, i.e., the energizing ray ispolarized elliptically instead of linearly. Such a depolarizationappears for a certain type of error, for example, in the case where themagneto-optical element possesses some double refraction or exhibitsVoight-effect, or particularly with multiple reflections within themagneto-optical medium, if non-ideal mirrors or non-ideal divider platesare utilized. Errors of this type manifest themselves, apart fromamplitude variations which can be balanced without major expenditures,by an additional phase displacement of the detector signals. Accordingto an important embodiment of the invention, the angle between thedirections of oscillation of the two partial beams of 1r/4 provided,i.e., 45 for the ideal case is varied by rotating the polarizers by apositive, or negative angle B until the phase displacement of thedetector signals has attained the desired value of 1r/2 i.e., 90. Agreat portion of the phase displacements caused by the abovementionederrors can be balanced by means of these expedients. A check to seewhether the two signals are in the optimum quadrature relationship canbe easily made, for example, by applying the two signals to thecoordinate amplifiers of an oscilloscope. In the optimum quadraturerelationship between the two signals, one will see a circle on thescreen.

Other advantages and features of the invention will become more apparentfrom the following description of one suitable embodiment thereof andfrom the accompanying drawings wherein:

FIG. 1 is a schematic representation of the optical arrangement up tothe photo-detectors for measuring a magnetic field in accordance withthe improved technique of the invention; and

FIG. 2 illustrates, in schematic form, the electrical circuit forevaluating the magnetic field from information passed to it from thephoto-detectors.

With reference now to the drawings, and to FIG. 1 in particular, it willbe seen that a light beam having an amplitude A is passed through amagneto-optical element 3 surrounded by a coil 3a carrying a current tobe measured. After emerging from the magneto-optical element 3, thelight beam is directed onto a 45 inclined divider plate 5 where it issplit into two partial beams 6 and 7 of about equal intensity. Thepartial beam 6 passes directly through the divider plate, its directionbeing unchanged, whereas the other partial beam 7 is reflected by anangle of relative to partial beam 6.

Before entering the magneto-optical element 3, the light beam 1 ispolarized linearly. Its state of oscillation is indicated in the circle2. The plane of oscillation of the light beam 1 is then rotated by theangle after passing through the magneto-optical element 3, if a magneticfield is produced in the element 3 by current flowing through coil 3a.The state of polarization of the beam after passing through themagneto-optical element 3 is indicated in circle 4.

After leaving the divider plate 5, the partial beams 6 and '7 areconducted respectively through polarizers, which are preferablypolarization filters, 8 and 9, and pass to their respectively associatedphoto-detectors l0 and 11. The direction of oscillation of filter 8,i.e., its pass-through direction, is approximately parallel to the planeof oscillation of the unrotated light, i.e., 0 1 B as indicated incircle 2. Filter 9 is rotated by an angle of about 1r/4 with respect tothe direction of oscillation of filter 8.

After the light has been transformed into cor responding electricalsignals in the photo-detectors 10 and 11, the signals arrive in formaccording to Equation (1) and (2) respectively via conductors 22, 23, atthe input amplifiers 12, 13 shown in FIG. 2. As a rule, the directcurrent portion of the signals is split off here. The outputs fromamplifiers 12, 13 are delivered respectively to multiplying mixer stages15, 16 which can be, for example, ring modulators, or four-quadrantmultipliers. In these mixer stages i equal to k, cos 24 (k being aconstant) is multiplied by a signal g equal Similarly, i equal to k sin2 (k being a constant) is multiplied by a signal g equal to cos 6- 01 4a), is the frequency generated in the local oscillator 14 andcorrespondingly phase-shifted by the phase-shifters 17,18 connectedrespectively to inputs of the mixer 15,16.

These products from the outputs of the mixer stages 15,16 are deliveredto, and added in, an adding unit 19 and one thus obtains (with k k, land without consideration of a phase constant) an electrical signalhaving the form:

G(t)=COS (w t 2d) (1)) and which can be easily frequency-demodulated ina conventional manner in the after-connected demodulator unit 21 afterbeing passed through a limiting amplifier unit 20.

In conclusion, the invention accordingly yields a high resolution of theelectronic system with a maximum yield of the optical system. I claim:

1. Apparatus for measuring the current flowing through a conductorcomprising a magneto-optical element including a coil through which thecurrent is passed to produce a corresponding magnetic field, means forpassing a beam of linearly polarized light through said magneto-opticalelement which effects a rotation of the polarization plane proportionalto the magnetic field, an optical divider into which said light beam isdirected after leaving said magneto-optical element, said dividerserving to split said light beam into two partial light beams, meansdirecting one of said partial beams through a first polarizer to a firstphotodetector, means directing the other partial beam through a secondpolarizer to a second photo-detector, said second polarizer transmittinglight polarized in a direction rotated by about 45 relative to that ofsaid first polarizer, multipliers individual to and receiving therespective outputs from said first and second photodetectors, a localoscillator having its output connected into said multipliers to mix therespective detector signals with a sinusoidal oscillation signal fromsaid oscillator, an adding member to which the outputs from saidmultipliers are connected, and a frequency demodulating device to whichthe output from said adding member is connected.

2. Apparatus as defined in claim 1 wherein at least one of saidpolarizers is rotatable whereby the phase shift between said detectorsignals can be varied to attain a value of exactly 3. Apparatus asdefined in claim 1 wherein the direction of polarization of said firstpolarizer is oriented parallel to the oscillation plane of the unrotatedlight.

4. Apparatus as defined in claim 1 and wherein phase shifting means areincluded in the connections between the output of said local oscillatorand said multipliers and which effect a relative phase shift of 1r/2 inthe oscillator output applied respectively to said multipliers.

5. Apparatus as defined in claim 4 wherein the output from saidoscillator to one of said multipliers includes a phase shifter of +1r/4and the oscillator output to the other multiplier includes a phaseshifter of -vr/4.

1. Apparatus for measuring the current flowing through a conductorcomprising a magneto-optical element including a coil through which thecurrent is passed to produce a corresponding magnetic field, means forpassing a beam of linearly polarized light through said magneto-opticalelement which effects a rotation of the polarization plane proportionalto the magnetic field, an optical divider into which said light beam isdirected after leaving said magneto-optical element, said dividerserving to split said light beam into two partial light beams, meansdirecting one of said partial beams through a first polarizer to a firstphoto-detector, means directing the other partial beam through a secondpolarizer to a second photo-detector, said second polarizer transmittinglight polarized in a direction rotated by about 45* relative to that ofsaid first polarizer, multipliers individual to and receiving therespective outputs from said first and second photo-detectors, a localoscillator having its output connected into said multipliers to mix therespective detector signals with a sinusoidal oscillation signal fromsaid oscillator, an adding member to which the outputs from saidmultipliers are connected, and a frequency demodulating device to whichthe output from said adding member is connected.
 2. Apparatus as definedin claim 1 wherein at least one of said polarizers is rotatable wherebythe phase shift between said detector signals can be varied to attain avalue of exactly 90*.
 3. Apparatus as defined in claim 1 wherein thedirection of polarization of said first polarizer is oriented parallelto the oscillation plane of the unrotated light.
 4. Apparatus as definedin claim 1 and wherein phase shifting means are included in theconnections between the output of said local oscillator and saidmultipliers and which effect a relative phase shift of pi /2 in theoscillator output applied respectively to said multipliers.
 5. Apparatusas defined in claim 4 wherein the output from said oscillator to one ofsaid multipliers includes a phase shifter of + pi /4 and the oscillatoroutput to the other multiplier includes a phase shifter of - pi /4.